docs: def _get_instance_dimensions(self, ds):
          '''
          Returns a list of dimensions marked as instance dimensions
  
          :param netCDF4.Dataset ds: An open netCDF dataset
  
          :rtype: list
          :returns: A list of variable dimensions
          '''
          ret_val = []
          for variable in ds.get_variables_by_attributes(cf_role=lambda x: isinstance(x, basestring)):
              if variable.ndim > 0:
                  ret_val.append(variable.dimensions[0])
          return ret_val